Field
The presently disclosed subject matter relates to a video projection apparatus. The video projection apparatus can be used as a pico projector, a head mount display (HMD) unit, a head up display (HUD) unit and the like.
Description of the Related Art
A video projection apparatus is constructed by a light source and an optical deflector using a digital mirror device (DMD), a liquid crystal on silicon (LCOS (trademark)) device or a micro electro mechanical system (MEMS) device.
In an optical deflector using a DMD or an LCOS device, spatial light is always incident from the light source onto the entire DMD or the entire LCOS device. As a result, a view field having a fixed horizontal angle and a fixed vertical angle can always be projected at a fixed screen. In this case, each element of the DMD or LCOS device is switched ON or OFF, so that ON-light reflected from the switched-ON element is used for projection, while OFF-light from the switched-OFF element is reflected or absorbed by an optical filter or a light absorber to dump the OFF-light. Thus, utilization of light is low.
On the other hand, in a MEMS optical deflector, a fixed screen is scanned two-dimensionally with a light ray (beam) generated from the light source which light beam is switched ON and OFF, so that utilization of light can be high.
However, since the MEMS optical deflector is susceptible to electromagnetic interference (EMI) noises and external noises by environmental factors such as the temperature and the humidity, a resonant frequency would be changed. As a result, it is difficult to accurately control a projected view field.
A prior art MEMS micromirror scanner has a photodetector disposed at an end shifted from the center of a projected view field to detect a rocking angle of a micromirror. Generally, the rocking angle of a micromirror depends on the temperature, i.e., the higher the temperature, the smaller the rocking angle. Therefore, the rocking angle can be adjusted on the basis of the timing position of the output voltage of the photodetector with respect to the center of the projected view field under the condition that the frequency of the drive voltage for the MEMS micromirror scanner is fixed at a definite resonant frequency of the micromirror (see: JP2007-86626A).
In the above-described prior art MEMS micromirror scanner, however, as stated above, the resonant frequency of the micromirror would be changed. Therefore, it is impossible to determine whether the deviation of the rocking angle is due to the deviation of the drive voltage for electrostatically driving the micromirror or due to the deviation of the resonant frequency of the micromirror.
In FIG. 16A, which shows an ideal state of the rocking angle of the micromirror, the rocking angle ϕ varies at a resonant frequency fr (=1/Tr). When ϕ=ϕpd, the photodetector generates an output voltage Vpd at time t1, so that the maximum rocking angle ϕ is a desired rocking angle ϕd. Also, in FIG. 16B, which shows a drive voltage deviation state where the drive voltage for the MEMS optical scanner is deviated to the lower side, when ϕ=ϕpd, the photodetector generates its output voltage Vpd at time t2 later than time t1, and also, the maximum rocking angle ϕ is smaller than the desired rocking angle ϕd. In this case, the drive voltage is controlled to be increased, so that time t2 is brought close to time t1. Thus, the maximum rocking angle ϕ is brought close to the desired rocking angle ϕd. On the other hand, in FIG. 16C, which shows a resonant frequency deviation state where the resonant frequency is fr′ (=1/Tr′) smaller than fr, when ϕ=ϕpd, the photodetector also generates its output voltage Vpd at time t3 later than time t1, and also, the maximum rocking angle ϕ is smaller than the desired rocking angle ϕd. Therefore, the drive voltage is controlled to be increased, so that time t3 is brought close to time t1. In this case, however, the maximum rocking angle ϕ is beyond the desired rocking angle ϕd.
Further, the electromagnetic interference (EMI) noises and external noises by environmental factors such as the temperature and the humidity would affect the operation of a control unit for controlling the micromirror scanner. Therefore, the above-described micromirror scanner would not be operated at its optimum resonant frequency of the micromirror. As a result, if the resonant frequency is deviated, the desired rocking angle of the micromirror would be unstable.